In a digital communication system, the transmission channel bandwidth is always limited. In order to increase system capacity or number of users as much as possible within the limited bandwidth, it is necessary to raise the channel bandwidth utilization. One of the important ways of doing so is by using a low speed speech encoding method to compress the source data. At present, the low speed speech encoding method is based on a fixed frame length. Based on the Internet protocol (IP) transmission, the data context of a speech signal is broken into one speech frame or multiple speech frames with a fixed bit number.
The working procedure, including transmitting and receiving, of a single frame or multiple frames with fixed length is as follows. At the transmitting end, first determine speech characteristics of an inputted speech signal with pulse code modulation (PCM). Then, determine encoding bit rate according to the characteristics of the inputted speech signal and the frame length and make compressed encoding. Finally, the compressed encoded single frame or multiple frames of the speech signal are assembled into TCP packets based on the transport control protocol (TCP) or UDP packets based on the user datagram protocol (UDP) and TCP packets or UDP packets are assembled into IP packets based on the Internet protocol (IP). The IP packets are sent to the receiving end through channels. At the receiving end, fist the packet is disassembled based on the IP, and then on the TCP or the UDP. Then, whether the packet contains an error or delay, and whether there are packets lost, are adjudged. If there is no such abnormal situation, then the packet is normally decoded and recovered to a speech signal with PCM to output. Otherwise, the speech signal with PCM is outputted after a pre-decoding of a signal frame or multiple frames.
In the above procedure, no matter whether an IP packet is a single frame or a multiple frame packet, since a fixed frame length is used, the encoding efficiency is low, and it is difficult to decrease the bit rate further. Furthermore, when an IP packet is a multiple frame packet, in situations of packet error, loss and delay, it is difficult to make any processing for signal decoding.